Can I use a plastic container that is not labeled microwave-safe if it has a recycling number 5?

Not necessarily. The recycling number tells you the type of plastic, but it does not guarantee that the manufacturer tested it for microwave use. A container with a 5 may be microwave-safe, but only if it also has the microwave-safe symbol. Without that symbol, the container could still warp or leach chemicals when heated. When in doubt, transfer food to glass or ceramic.

Is it safe to microwave food in a container that has a small crack?

No. Cracks, chips, or scratches create weak points where the plastic can break down under heat, potentially releasing chemicals or allowing food to seep into the material where bacteria can grow. Replace any container with visible damage before using it in the microwave. The same applies to glass containers, which can shatter if the crack weakens the structure.

Why do some microwave-safe containers feel hot when I take them out, while others stay cool?

This happens because the food heats up and transfers heat to the container through conduction. A container that feels hot is normal if the food inside is very hot. However, if the container feels hot immediately after heating while the food is still cool, it may not be microwave-safe. That indicates the container is absorbing microwaves instead of letting them pass through to the food.

Can I use the same plastic container for freezing and then microwave it later?

Only if the container is explicitly labeled as both freezer-safe and microwave-safe. Freezing and microwaving cause different types of stress on plastic. A container that is microwave-safe but not freezer-safe may become brittle in the cold and crack when heated. Always check both labels, and if you are unsure, transfer frozen food to a glass container before reheating.

How to Check If a Container Is Microwave-Safe and Use It Without Risk

Updated September 19, 2026 1204 words 5 min read

Look for the Microwave-Safe Symbol Before You Assume

The first step is to find the microwave-safe symbol on the container. It usually appears as a small microwave icon with wavy lines, or a triangle made of three arrows with the number 5 inside. That symbol is your green light. If you see it, the container has been tested for microwave use and should not warp or leach harmful chemicals under normal heating conditions.

If the symbol is missing, do not assume the container is safe. Many storage containers are marked on the bottom, but the print can wear off over time. Turn the container over and check the base, the lid, and the rim. If you cannot find any marking, treat it as not microwave-safe until you can confirm otherwise.

Even if the symbol is present, remember that it applies to the container as manufactured. A container that was microwave-safe when new may not stay that way after years of use, cracks, or exposure to high heat. Replace containers that show signs of wear before putting them in the microwave.

Use the Cup-and-Spots Test for Unmarked Containers

If you have a container without any marking, you can do a simple home test. Place the empty container in the microwave along with a microwave-safe mug filled with water. Heat for one minute on high. If the container stays cool and the water heats up, it is probably microwave-safe. If the container feels warm or hot, it is absorbing microwaves and should not be used.

This test works because microwave-safe materials allow microwaves to pass through without heating the material itself. A container that heats up is absorbing energy, which could lead to warping, melting, or the release of chemicals. Do this test only with an empty container—never test a container with food in it, because the food could mask the container's temperature.

Keep in mind that this test is not a guarantee of long-term safety. It only tells you whether the container reacts to microwaves at that moment. A container that passes the test today might fail later if it develops cracks or scratches. Repeat the test periodically, especially if you use the container frequently.

Check the Recycling Number to Identify Problem Plastics

Flip the container over and look for the recycling triangle with a number inside. Numbers 1, 2, 4, and 5 are generally considered safe for microwave use when the container is marked microwave-safe. Number 5 (polypropylene) is the most common for food storage because it withstands heat well and resists staining.

Avoid using plastics with numbers 3 (PVC), 6 (polystyrene), or 7 (polycarbonate) in the microwave. Number 3 may release phthalates, number 6 can melt and leach styrene, and number 7 often contains bisphenol A (BPA) unless it is explicitly labeled BPA-free. These plastics are not designed for high heat and can deform or transfer chemicals to your food.

If you are unsure about a plastic's number or cannot find the recycling symbol, do not use it in the microwave. Even if the plastic is BPA-free, the heat can break down the polymer structure. When in doubt, transfer food to a glass or ceramic container instead of risking your health or your appliance.

Test the Lid and Vent It Properly Before Heating

Lids are often the weak point in microwave-safe storage containers. Even if the base is microwave-safe, the lid may be made of a different plastic that is not. Check the lid for its own microwave-safe symbol. If it has none, remove the lid before heating and cover the container with a microwave-safe plate or paper towel instead.

When the lid is marked safe, do not snap it shut. Microwave heating builds up steam, and a sealed lid can burst, causing hot food to splatter and burn you. Loosen the lid or leave it slightly ajar to allow steam to escape. Alternatively, use the vent hole if your container has one—many modern lids include a small steam vent specifically for this purpose.

If you are reheating foods with high liquid content, like soups or stews, stir the food halfway through. This helps distribute heat evenly and prevents cold spots where bacteria can survive. Let the container sit for one minute after heating, as the temperature continues to even out during that time.

Avoid Heating Greasy or Sugary Foods in Plastic

Even microwave-safe plastic can be problematic with foods that have high fat or sugar content. Fats and sugars reach temperatures much higher than the boiling point of water, and this can cause the plastic to soften, melt, or leach chemicals into your food. Examples include butter, bacon, chocolate, and syrups.

For these foods, transfer them to a glass or ceramic container before microwaving. Glass and ceramic do not absorb microwaves the same way, so they handle high temperatures without breaking down. If you must use plastic, choose a container labeled as safe for high-heat use, and reduce the power level to 50% to lower the maximum temperature.

Also, avoid microwaving plastic containers that have scratches, cracks, or discoloration. These imperfections can harbor bacteria and make the plastic more likely to release chemicals. When a plastic container shows signs of wear, retire it from microwave duty and use it only for dry storage or discard it.

Use a Paper Towel or Microwave-Safe Cover to Prevent Splatters

Covering your food while reheating serves two purposes: it keeps your microwave clean and helps the food heat more evenly. A microwave-safe lid, a paper towel, or a microwave-safe plate placed upside down over the container works well. Avoid using plastic wrap unless it is labeled microwave-safe, and even then, leave a corner open for steam to escape.

When using a paper towel, make sure it is plain white and not printed with metallic designs. Some recycled paper towels contain metal fragments that can spark in the microwave. The paper towel also helps trap moisture, preventing your food from drying out. Do not use wax paper or parchment paper that is not labeled for microwave use.

After heating, remove the cover carefully. Lift the side farthest from your face first to let steam escape safely. This simple habit reduces the risk of steam burns, which are more common than burns from the hot container itself. Let the food stand for the recommended time before eating to allow heat to distribute evenly.

Know When to Replace Your Microwave-Safe Containers

Microwave-safe labels do not last forever. Over time, the heat, food acids, and detergents can degrade the plastic. If a container becomes cloudy, warped, or develops a lingering odor, it is time to retire it. These signs indicate that the material has broken down and may no longer be safe for microwave use.

Glass containers can last much longer, but they are not indestructible. Check for chips, cracks, or stress fractures, especially around the rim. A damaged glass container can shatter when heated, sending shards into your food. Replace glass containers that show any damage, and avoid sudden temperature changes, such as moving them from the freezer directly to the microwave.

As a general rule, replace plastic containers every six to twelve months if you use them regularly. If you are unsure whether a container is still safe, err on the side of caution and use a different one. The cost of a new container is far less than a trip to the emergency room or the risk of chemical exposure.

Frequently asked questions

Can I use a plastic container that is not labeled microwave-safe if it has a recycling number 5?: Not necessarily. The recycling number tells you the type of plastic, but it does not guarantee that the manufacturer tested it for microwave use. A container with a 5 may be microwave-safe, but only if it also has the microwave-safe symbol. Without that symbol, the container could still warp or leach chemicals when heated. When in doubt, transfer food to glass or ceramic.
Is it safe to microwave food in a container that has a small crack?: No. Cracks, chips, or scratches create weak points where the plastic can break down under heat, potentially releasing chemicals or allowing food to seep into the material where bacteria can grow. Replace any container with visible damage before using it in the microwave. The same applies to glass containers, which can shatter if the crack weakens the structure.
Why do some microwave-safe containers feel hot when I take them out, while others stay cool?: This happens because the food heats up and transfers heat to the container through conduction. A container that feels hot is normal if the food inside is very hot. However, if the container feels hot immediately after heating while the food is still cool, it may not be microwave-safe. That indicates the container is absorbing microwaves instead of letting them pass through to the food.
Can I use the same plastic container for freezing and then microwave it later?: Only if the container is explicitly labeled as both freezer-safe and microwave-safe. Freezing and microwaving cause different types of stress on plastic. A container that is microwave-safe but not freezer-safe may become brittle in the cold and crack when heated. Always check both labels, and if you are unsure, transfer frozen food to a glass container before reheating.